This invention relates to electronic solid state devices, particularly but not exclusively photosensitive solid state devices such as solid state imaging devices and photodetector devices.
In many electronic applications there exists the requirement for an element the electrical conductivity of which can be controlled by means such as light radiation and capable of operating with a high speed of response and providing a high gain. For example, in imaging applications there exists a requirement for a photoconductive element having a high gain and a high speed of response. Conventional photoconductors, such as cadmium sulphide, have a relatively high grain but a relatively slow speed of response because the gain mechanism is due to carrier lifetime being increased by trapping effects.
During the last twenty years a considerable effort has been devoted to replacing the conventional vacuum tube image intensifier with an all solid state equivalent. The potential advantages are lower weight, cheaper components, lower operating voltages, more rugged structures and the possibility of large area devices in the form of thin panels. The basic features of all image intensifiers are a detector, an amplifier and a display. In modern vacuum tubes these are represented respectively by a photocathode, a secondary emission multiplier or an accelerator, followed by a phosphor screen. Solid state image intensifiers have normally been based on a secondary photoconductor as the detector and amplifier followed by an electroluminescent display. The solid state equivalent of secondary emission multiplication is avalanche grain and although this has been considered for the role of amplifier, in practice sufficiently high gains have not yet been achieved with adequate stability. Although useful image storage devices have been made using the photoconductor plus electroluminescence system, the response time, particularly at low light levels and high gains is too long for viewing moving objects.
Although hitherto some semiconductor devices have been produced in the form of an inhomogeneous body comprising grains of semiconductor material, for example a powder layer in a suitable binder, this invention is based on a new concept involving the recognition that if in an inhomogeneous body comprising grains of semiconductor material the formation of the body is effected in such manner as to control the nature of the grain to grain contacts with the deliberate addition in the body of certain conductivity modifying species at least in the vicinity of said grain to grain contacts then it is possible to form a structure in which field effect control of the conductivity can be obtained, said control being at least in part dependent upon external influences such as incident radiation, atmosphere, temperature, and incident electrical charge.
The invention is further based on the recognition that the employment of such field effect control of conductivity can lead to a new approach to image detection and amplification which provides the possibility of high gain, high detection efficiency, fast response time, variable integration time, and a wide range of spectral sensitivity.
The type of gain which occurs in multiplying devices such as electron multipliers and avalanche diodes is essentially instantaneous. An input current generates a real increase in the number of carriers available for conduction through some external load. Photoconductive gain on the other hand arises when the lifetime of one or both of the photogenerated carriers constituting the input of primary current is sufficiently large that an effective increase in carriers available for conduction occurs. If both carrier are simply collected by the electrodes, then one electron effectively traverses the external circuit and the gain is unity. For gains greater than unity at least one carrier must be replenished at an electrode, and the lifetime of one of these carriers must be greater than its transit time between the electrodes.
The said lifetime value can be increased by decreasing the recombination probability of the photoexcited electron-hole pairs. Materials with a suitably high value of lifetime for electrons have hole traps which when full exhibit a low capture cross section for electrons. These traps also reduce the effective hole mobility which reduces recombination at the electrodes.
If no other processes were involved then the decay time of the photoconductor after ceasing illumination would be the said carrier lifetime value. In practice much longer decay times are observed and are attributed to electron traps. If deep enough these may control the rate of recombination of free electrons rather than the hole capture process. They may also have the effect of reducing the effective mobility and thus the gain G. As a result values of gain G .perspectiveto. 10.sup.4 have been observed for cadmium sulphide with decay times of several seconds, whereas from the geometry, mobility and voltage much higher gains should have occurred if the decay time corresponded to the carrier lifetime.
The present invention arises in part as a result of the basic concept of photoconductive gain being accepted, but measures being effected to provide a means of externally and continuously controlling the critical parameter, the carrier lifetime.
The requirements apart from gain of a photoconductor system may be summarised as follows:
a. a high efficiency of absorption of incident photons,
b. a high efficiency of separation of photoexcited electron-hole pairs in short time ts,
c. a high mobility for one of the photoexcited carriers,
d. ohmic contacts,
e. efficient removal of the mobile carrier in a very short tim (t.sub.r) after a specified period of time (t.sub.i),
f. accurate external control of this integration time (t.sub.i),
g. t.sub.i should be &gt;&gt; t.sub.s and t.sub.r + t.sub.i should be &lt;the integration time of the eye (t.sub.e) for viewing dynamic images.
Requirements (c) and (e) also require that deep bulk electron traps should be absent.
The expression for the gain is: EQU G = t.sub.i /t.sub.t
where t.sub.t is the transit time between the electrodes. The effective response time is equivalent to the integration time t.sub.i and may be varied over the range t.sub.r &lt;t.sub.i &lt;t.sub.e for viewing dynamic images. For image storage applications t.sub.i may be greater than t.sub.e.
For meeting the above requirements, in particular the critical stage (c) structures falling within the following two classes are relevant. These are:
1. Those which are normally non-conducting in the dark and in which photo-generation gives rise to a non-equilibrium conducting state, and
2. Those which are normally conducting in the dark and which are placed in a non-equilibrium non-conducting state prior to photogeneration.
Normal photoconductors are in the first class and requirement (e) above could conceivable be achieved by field- or photo-emptying of trapped holes. It may also of course be achieved by elimination of the electron traps alone.
The second class involves some form of depletion of the free carriers by a stored charge to achieve the non-conducting state, while the progressive neutralisation of this stored charge by photogenerated carriers give rise to the photosensitivity. Requirement (c) may then be achieved by replacing the stored charge from an external source at specified intervals. The present invention is also based upon the discovery that it is in the second class of devices that charge storage effects already known in monocrystalline semiconductor device technology in junction field effect transistor (JFET) structures can be utilised to advantage in an entirely different area wherein the active body of the device comprises grains of semiconductor material.